Waveform generator



June'15, 1955 Y A. HOPENGARTEN E'rAL 3,139,755

WAVEFORM GENERATOR 2 sheets-sheen Filed Oct. 19. 1961 F/q. Z.A

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June l5, 1965 A. HoP-:NGARTEN la'rAl. 3,139,756

WAVEFORM-GENERATOR 2 Sheets-Sheet 2 Filed Oct. 19, v1961 Fmg. 5.

omnia/M United States Patent O 3,189,756 WAVEERM GENERATR AbramHopengarten, Lafayette Hiii, and Paul G. Wolfe, reland, Pa., assignors,by rnesne assignments, to Ihiico Corporation, Philadelphia, Pa., acorporation ot Deiaware Filed Oct. 19, 1961, Ser. No. 146,224 9 Claims.(Cl. 367-885) This invention relates to a circuit for generating atime-varying electric current in a load. More particularly it relates toa transistor circuit for generating a sweep current in a deiiection yokeof a television receiver.

To reduce the size, weight and power consumption of televisionreceivers, transistor circuits have been substituted for vacuum-tubecircuits in such receivers. The substituted transistor circuitsfrequently have been designed merely to perform the same functionsheretofore performed by the corresponding vacuum-tube circuits. Thus atypical prior-art transistorized vertical sweep system of a televisionreceiver, designed in imitation of a vacuum-tube sweep system, comprisesa transistor oscillator which generates a periodically recurringsawtooth voltage, a transistor driver st-age which converts thissawtooth voltage into a sawtooth current of low intensity and atransistor output stage which ampliiies the driver output current to anintensity suiiicient to cause deflection of the cathode ray beam of thepicture tube across its entire screen when supplied to the Verticaldeiiection yoke of the receiver.

Such transistorized substitutes for vacuum tube circuits often areunnecessarily complex because they fail to take advantage of thecurrent-amplifying properties and non-linearities peculiar totransistors. For example the above-described sweep circuit requiresthree transistors rather than only two because vone transistor isemployed, unnecessarily, to generate a sawtooth voltage waveform whichthen must be converted by the succeeding transistor into a sawtoothcurrent waveform. Moreover, because a transistor is not adapted toamplify linearly an amplitude-varying voltage, various distortions areintroduced into the output current waveform by the non-linearitiesinherent in the transistor driver stage which converts the sawtoothvoltage into a sawtooth current. To eliminate these distortions,relatively complex feedback and waveshaping networks have been employedheretofore. All this complexity causes the transistor circuit to beexpensive and relatively susceptible to component failures, and torequire an unnecessarily large amount of power for operation.

Accordingly an object of the invention is to provide an improvedtransistor circuit for generating time-varying currents.

Another object is to provide such a transistor circuit, which utilizesadvantageously both the current-amplifying properties and non-linearityof a transistor in generating the current waveform.

Another object is to provide a sweep-current generating circuit whichrequires only two transistors.

The foregoing objects are achieved utilizing a transistor having abase-emitter resistance which decreases in response to an increasingbase-emitter forward-biasing voltage and a current gain which decreasesin response to an increasing collector current. To produce a linearlyincreasing collector current which may be supplied to a verticaldeflection yoke as a sweep signal, the transistor is supplied with aprogressively increasing base-emitter current, the waveform of whichcurves upwardly suficiently to compensate for the decrease in currentgain ice of the transistor. This base-emitter current is supplied by aresistor-inductor charging circuit having a resistor connected in shuntwith the base-emitter path of the transistor. As the intensity of thecurrent ilowing through the charging circuit increases, the increasingforward-biasing voltage applied by the resistor between the base andemitter of the transistor reduces the resistance of its base-emitterpath. As a result an increasingly large portion of the increasingcharging current flows through the base-emitter path of the transistor.Hence the waveform of this portion of the charging current curvesupwardly even though the waveform of the charging current curvesdownwardly. Because the transistor is operated in its unsaturatedcondition, the collector acts as a high-impedance current source. Hencethe waveform of its output current, when supplied to a relatively lowimpedance load such asthe vertical deflection yoke of a televisionreceiver, is substantially unaffected by the electrical characteristicsof the load.

Other advantages and features of the invention will become apparent fromthe following description taken in connection with the accompanyingdrawings in which FIGURE l is a schematic diagram of a simplifiedcircuit used to explain the invention;

FIGURES 2 and 3 are graphs of pertinent non-linear parameters of atransistor;

FIGURE 4 is a graph relating to the operation of the circuit of FIGUREl;

FIGURE 5 is a schematic diagram of an embodiment of the invention;

FIGURE 6 is a graph of the collector characteristics or" a transistor;4and FIGURE 7 is a schematic diagram of another embodiment of theinvention.

Referring to FIG. l, transistor Z2, which is of the p-n-p type andconnected in the common-emitter congurati-on, is energized by a source18 of unidirectional current having its positive pole connected directlyto emitter 24 and its negative pole connected to collector 32 by way ofa direct-current-conductive load 3). Capacitor 2@ bypasses battery 13.Load 3@ may be the vertical deflection yoke of a television receiver.

A linearly increasing current is generated in load 3i) in response to anincreasing current of upwardly curving waveform owing in thebase-emitter path of transistor 22. A base-emitter current having thiswaveform is produced by the co-action of the base-emitter path of thetransistor and a resistor-inductor charging circuit to which this pathis connected. The charging circuit comprises source 18, resistor 1li,switch 12, Variable resistor 14 and inductor 16 connected in seriesrelationship. Resistor 1t) also is connected in shunt with thebase-emitter path of transistor 22. Closure of switch 12 initiates theow of the base-emitter current.

To limit the voltage developed by inductor 16 across switch 12 whenswitch 12 is opened, capacitor 34 is connected in shunt with inductor16. To damp the oscillatory current generated in inductor 16 andcapacitor 34 when switch 12 is opened, a damping resistor 36 is shuntedacross the capacitor and inductor.

Transistor 22 exhibits two kinds of non-linearity-a variablebase-emitter resistance and a variable current gain-which affect theWaveform of the current supplied to load 3b. According to the invention,the variable base-emitter resistance is used to compensate for thevariable current gain. FIG. 2, which is a plot of the static resistanceof the base-emitter path of a Philco Type 2N386 transistor versus itsforward-biasing baseemitter voltage for a constant reverse-biasingcollector- Vbase 26 and emitter 24 of transistor 22.

emitter voltage of -2 volts, shows that the static input resistance ofthe transistor decreases by more than an order of magnitude as thebase-emitter voltage is varied from about 6.2 volt to about 0.6 volt.FIG. 3 which is a plot of the D.C. beta of the Type 2N386 transistorversus its collector current for a reverse-biasing collector-emittervoltage of -2 volts, shows that beta fails as the collector currentrises. With switch 12 open, no substantial current flows in any branchof the circuit. When switch 12 is closed, a current I of progressivelyincreasing intensity tiows from source 18 through inductor 16. Thiscurrent produces a voltage across resistor which is applied in aforward-biasing polarity between In response to .this voltage thebase-emitter path of the transistor becomes conductive. Thereafter thecurrent I from source 18 flows in part through resistor 10 as a currentcomponent Il and .in part through the base-emitter path of transistor 22as a current component I2. As the intensity of .current I increases, theintensity of component Il also increases. plied between base 26 andemitter 24 rises. As a result the base-emitter resistance of transistor22 falls (see FIG. 2). Since the value of resistor 10 remains constant,the ratio 12/11 increases. Consequently the waveform of the currentcomponent I2 lowing through the base-emitter path of transistor 22curves upwardly (sec curve 6u, FIG. 4) even though the waveform of thetotal current I curves downwardly (see curve 58, FIG. 4). Curve 62 (FIG.4) shows the waveform of the current component I1 flowing in resistor10. The amount of curvature of the waveform of component I2 can beadjusted by appropriate adjustment of the values of resistor 1li orresistor 14 or both.

Were transistor 22 a device having a constant beta over its entire rangeof collector current, a current having a waveform identical to thatshown at 60 would flow through collector 32 and be supplied to load 30.Such an upwardly curving waveform would not necessarily beY a desirablesweep waveform since it would cause the beam of the cathode ray tube tobe deectedV with inconstant velocity across the screen. However, asdiscussed hereinbefore, the beta of the transistor decreases as itscollector current increases. Since the intensity of the collectorcurrent flowing in transistor 22 is the product of its base current I2and the value of its beta, the waveform of this collector current may bemade linear by appropriately shaping the upward curvature of the basecurrent I2 to compensate for the downward curva ture of the betacharacteristic. This linear collector current waveform is shown in FIG.4 at 64. Transistor 22 is operatad under unsaturated conditions.Therefore its collector 32 is a high-impedance current source. Thecollector current flows through load 39 without substantial change inits waveform. When switch 12 is opened,

the current in inductor 16 rapidly decreases in wellknown manner. byline 66.

FIG. 5 shows an astable waveform generating circuit according to theinvention. In addition to the components of the simplified circuit ofFIG. 1, the circuit of FIG. 5 comprises both a feedback network forcausing a repetitive current waveform to be generated at a givenfrequency and .means for controlling the maximum intensity of thecurrentgwaveform generated in load 30. A transistor 70 is used as theswitch 12.

More particularly transistor '70, like transistor 22, is of the p-n-ptype. Ernitter 72 of transistor 70 is connected to resistor 10, andcollector 74 thereof is connected to resistor 14. To control the maximumvoltage supplied to the lseries charging circuit including resistor 10,the emitter-collector path of transistor 70, resistor 14 and inductor16, a variable resistor 76 is connected between inductor 16 and a pointat reference potential. A

This decrease is represented in FIG.. 4

Hence the forward-biasing voltage ap- 4 no longer operate undersaturation conditions.

capacitor 78 is connected across resistor 76 to lay-pass alternatingcurrents.

To permit a feedback lsignal to be derived therefrom, inductor 16 ismade the primary winding of a phasereverisng transformer 32 having asecondary winding 84. To apply the feedback sign-al between emitter 72and base 81B of transistor 70, one terminal of secondary winding S4 isconnected directly to base 3% and the other terminal thereof isconnected by way of a capacitor 86 to emitter- 72. To supply D.-C. basecurrent to base E@ and control the repetition rate of the circuit, avariable resistor $8 is connected in series relationship with variableresistor 76 and secondarywinding 84.

The operation of the circuit ofFIG. 5 is as follows. Whenswitchingtransistor 7@ is driven into a collectorcurrent saturation condition byapplying a forward-biasing voltage between base 80 and emitter 72, asdescribed hereinafter, a current of progressively increasing intensityfiows from source 1d through the parallel combination of resistor 1@ andthe base-emitter path of output transistor 212, and thence through theemitter-collector path of switching transistor 7G, resistor 14 andinductor 16. This current produces the desired linearly increasingcurrent through load 30 in the manner already described in connectionwith the embodiment of FIG. l. In addition the increasing currentowingthrough inductor 16 induces a voltage across secondarywinding 54 in apolarity such as to forward-bias theba'se-emitter path of transistor 70.This voltage is suihciently high to maintain transistor 70 insaturation. As a result the impedance of the emitter-collector path'oftransistor 7i) remains very low and the current supplied by source 13continues to increase. This ,increase ends when the collector current ofswitching transistor 711 is suliiciently large with respect to its basecurrent that transistor 7i) can FIG. 6, comprising plots of collectorcurrent versus collector voltage for various constant base currents fora Philco Type 2N224 transistor connected in the common-emitterconfiguration, shows that foreach base current there is a first range ofcollector voltages for which the collector current rises as thecollectork voltage rises (the region of saturated operation), andasecond range of collector voltages for which the collector currentremains substantially constant even though the collector kvoltage rises(the region of unsaturated operation).

In the circuit of FIG. 5, the collector current of transistor 7 0 risesto the maximum value which transistor 76 can supply in View of its basecurrent, in a time determined by the rate of increase of current throughinductor 16. Then thecurrent supplied by collector 74 to inductor16becomes substantially constant. This constant current induces no voltageacross secondary winding $4; as a result the base current supplied toswitching transistor decreases. In response to this decrease, thecollector current supplied by transistor 76 -to inductor 16 decreases.This ldecreasing current induces` `across secondary winding 84 areverse-biasing voltage ywhich cuts oif switching transistor 70.

Because switching transistor '70 is cut olf, its emittercollector pathhas -a Very high impedance. Because mag- 'neticenergyV is stored ininductor 16, an oscillatory current tends to Vlow between inductor 16and capacitor 34. Resistor 36 damps this oscillation. Initially areversebiasing voltage is induced across secondary winding 84 by thecurrent flowing in inductor 16. Thereafter, at a time determined inwell-known manner by the values of inductor 16, capacitor 34 andresistor 36, the voltage induced across winding S4 reverses polarity.That is, a forward-biasing voltage of progressively increasing valueappears across winding 84. This voltage rapidly drives switchingtransistor 76 into saturation. As a result, the impedance of theemitter-collector path of transistor 7u falls and damps out furthercurrent oscillations in inductor 16 and capacitor 34. At this point intime, the circuit of FIG. 5 has completed one full cycle of operation,and a new cycle begins.

IResistor 818 controls the rate at which the foregoing repetitive cycleoccurs by controlling the base current Iflowing into .switchingtransistor 70. By controlling this base current, resistor 88 determinest-he value of collector current for which switching transistor 70 ceasesto operate under saturation conditions and initiates the yback portionofthe repetitive cycle.

Resistor 7.6 controls the .total voltage applied across the seriescircuit comprising inductor 16, resistor 14, the emitter-collector pathof transistor 70, .and the parallel combination of resistor 1() and thebase-emitter path of transistor 22. Hence it controls the maximumintensity of the current flowing through inductor 16 as well as its rateof change.l However, variation of resistor 76 does not changesubstantially the repetition rate of .the circuit. More particularly,.as the value of resistor 76 is decreased, a larger voltage is appliedacross the above-described series circuit. This larger voltage `causesthe current flowing through the series circuit to increase more rapidly.This increase in rate tends to increase the repetition rate of thesystem of FIG. 5 by causing transistor 70 to corne out .of saturationmore rapidly. However, decreasing the resistance of resistor 76 alsoincreases the base current supplied to transistor 70. Hence .thecollector current at which transistor 70 comes out of saturation alsorises and additional time is required for the collector current toattain this new higher maximum value.

As aforementioned, the circuit of FIG. 5 may be used in a televisionreceiver as a sweep oscillator and sweep output circuit, e.g. as thevertical oscillator .and output stage thereof. In such an application,load 30 is the vertical deection yoke of the receiver. The circuit isheld in synchronis-m by supplying vertical synchronizing pulses ofpositive polarity to base 80 via terminal 106 and winding 84, and thevalue of resistor 88 is adjusted s-o that the circuit has a naturalrepetition rate slightly lower than lthe repetition rate of the verticalsynchronizing pulses. When a vertical sync pulse is applied to base 80,it reduces the base current of transistor 70 to a value such thattransistor 70 no longer operates under saturation conditions. Hence .thepulse initiates the yback portion of the repetitive cycle. The durationof the yback portion is controlled .by the values of inductor 16,capacitor 34 and resistor 36.

llFlIG. 7 shows a vertical sweep generating circuit simil-ar to that ofFIG. 5 but incorporating features no-t found in `the circuit of FIG. 5.Only those portion-s of FIG. 7 which differ from FIG. 5 will bedescribed. In this circuit b-ase bias is supplied to transistor 70 froma lowimpedance source comprising resistors 110 and 112, thereby vtoenhance the temperature stability of the circuit. Resistors v1110 and.112 are connected between variable resistor 88 and the positiveterminal of source 18. A resistor 114 connects the junction 116 ofresistors 110y and 112 to base Sli. One terminal of secondary winding 84also is connected to junction 116. Another resistor 1118 couples theother terminal of winding 84 to base 80 via a blocking capacitor 120.Resistors 114 and 118 .and capacitor 120 constitute a differentiatingnetwork which sharpens the pulses supplied to base S0 by secondarywinding S4. Such sharpening increases the frequency stability of .thecircuit.

To inhibit spurious triggering by noise pulses, a syncpulse gatecomprising diode 122 is provided.l Base Si? of switching transistor 70is connected to cathode 124 of diode 122. A resistor-capacitorintegrating network 126 is coupled by a blocking capacitor 128 t-o theanode 130 of diode 122. Network 126 integrates `the separated syncpulses supplied thereto, thereby deriving in conventional manner .thevert-ical .sync pulses. -It supplies these pulses to anode 130 viacapacitor 128, In addition a resistor 132 and a capacitor 134, seriallyconnected between the collector 32 of output transistor 22 and anode 130of diode 122, supply a positive-going replica of Ithe sweep voltage toanode 130 as a gating voltage. A resistor 1-36 of high value provides adischarge path for capacitors 128 and .1.34.

During the interval between the trailing edge of one sync pulse and atime slightly before the leading edge of the next sync pulse, diode .122is reverse-biased by supplying a positive potential to its cathode 124via resistor 114 and supplying a potential negative with respect tocathode potential to its anode 130. The negative potential is suppliedby capacitor 113-4 which is charged by the gating voltage during timeswhen diode 122 is forward biased. Hence during said interval, diode 122has a high impedance and effectively prevents transmission to base ofnoise pulses appearing at anode 131). However, during said interval .thesawtooth gating voltage supplied to anode 130 becomes progressively morepositive, `and at a time slightly before the next `sync pulse thisgating voltage is .suliiciently positive to forward-bias diode 122.Under these conditions, diode 122 4applies the next sync pulse to .base80 without substantial attenuation and triggers the circuit into itsyback condition in the manner discussed in connection with FIG. 5. Tostabilize the operation ofthe output transistor 22, a low-valuedresist-or 138 is connected in series with its emitter 26.

To permit more precise shaping of the Waveform of the current suppliedby collector 32 to yoke 30, a negative feedback network is provided.This network comprises a resistor 142 and .a capacitor 144 connectedserially be- .tween deection yoke 30 and emitter 24. It also comprises avariable resistor 146 and a capacitor 14S connected serially betweenbase 26 and the junction 150 of capacitor 144 and resistor 142. Inoperation, the feedback voltage is-developed across resistor 142 by theflow of deilection cur-rent therethrough. In response to this voltage afeedback current is supplied to base 26 of transistor 212. rTheintensity and waveshape of this current depends in well-known manner onthe respective values of resistor 146 and capacitor 148. The current hasa sense such as to subtract trom the current supplied to base 26 by.transistor 70.

A transformer 152 having a primary winding 154 and a secondary winding156 is provided to derive a signal for cutting ott the cathode-ray beamof the picture tube during 4the retrace period. Primary winding 154 isconnected 4between collector 32 and the point at reference potential andprovides .a D.C. return for collector 32. Secondary winding 156 isconnected to circuitry of con- -ventional form which in response to eachilyback pulse supplied thereto by transformer 154, develops and appliesto the cathode of the picture tube a positive pulse of voltage whichcuts off the beam of the picture tube during the ily/back period.

The operation of the circuit of FIG. 7 is substantially :the same asthat of FIG. 5 except in the respects already noted. Accordingly nofurther discussion thereof is deemed necessary.

Typical values for the component-s ofthe circuit of FIG. 7 are asfollows:

Transistor 22 Type 2N1073. Transistor 70 Type 2N224. Resistor 10 47ohms. Resistor 76 3 kilohms. Resistor S8 2. kilohrns. Resistor 470 ohms.Resistor 112 1.8 kilohms. Resistor 114 2.2 kilohms. Resistor 113 lki-loh-m. Resistor 132 100 kilohms. Resistor 136 3.3 megohms. Resistor138 1 ohm. Resistor 142 do.

Resistor 146 250 ohms.

Resistors 158 1.8 kilohms each.

yot about l henry with milliam- Iperes D.C. owing therethrough. Theprim-ary-to-secondary turns ratio is 8.451141.

Transformer 152 Primary winding 154 had an inductance of about 0.6henry.

Source 18 18 volts.

, 2O The foregoing values are only exemplary and the invention is notlimited thereto.

In the embodiments discussed above, the transistors are all of the p-n-ptype. However, transistors of the n-p-n type may be substitutedtherefor. When this is done, the respective polarities of -the supplyvoltage and synchronizing pulses are reversed. In addition, in the iarrangement of FIG. 7, the poling of diode 122 is reversed.

While, in the foregoing embodiments, component Vhas been shown as .aresistor, di-rect-current-conductive elements having reactivecharacteristics can be used in place of or in combination with resistor10 to achieve output current waveforms not obtainable where the currentdivision between element 10 and the base-emitter path of transistor 22is determined solely by their relative resist-ences. Similarly inductor16 can be replaced by lother reactive elements.

While various specific embodiments of the invention have beenillustrated anddescribed, it is to be understood that the invention isnot limited thereto but contemplates such modifications land furtherembodiments as may occur to those skilled in the art.

What we cla-im is:

1. A waveform generator compris-ing: .a transistor having an emitter, a'collector and a base, lthe base-emitter 45 path of said transistorhaving a resistance which changes in response to a changinglforward-biasing voltage applied between said emitter and said base anda current gain which changes in response to a change in the collectorcurrent of said transistor, a direct-current-conductive impedanceelement, means connecting said impedace element between said emitter andsaid base, a reactive element, a switch and me-an-s for supplying aunidirectional voltage, means connecting said impedance element, saidswitch, said reactive element, and said voltage-supplying means inseries relationship, and a load coupled to the emitter-collector path of:said transistor, said unidirectional voltage having `such polarity lasto supply to said iinpedance element, upon Iclosure of said switch, aiirst eur- 4rent portion sensed to develop across said impedance elementa voltage which forward-biases said base-emitter path of saidtransistor, thereby enabling a `second current portion to iiow from saidvoltage-supplying means through said base-emitter path of saidtransistor, said unidirectional -voltage having a value sufiicientlyhigh `that said `forward-biasing voltage can change by `an amountproducing substantial changes respectively in said resistance of saidbase-emitter path, said collector current and said current gain of saidtransistor, said impedance element having an impedancevalue such that asubstantial change in said resistance of said base-emitter path whenforward biased causes a substantial change in the ratio of therespective intensities of said first and said second current portions,and means for closing said switch for a time sumeiently long for saidforward-biasing voltage to change in value by 2in-amount producing saidrespective substantial changes in said resistance of said base-emitterpath, said collector current and said current gain of said transistor. Y

'2. A waveform generator according to claim 1 wherein saiddirect-current-conductive impedance element is a resistive element.` V3. A Waveform generator 'according to claim 1, wherein said base-emitterpath of said transistor has a resistance which decreases in responseV.to an increasing for-wardbiasing voltage applied between said emitterand said base, said `direct-current-conductive impedance element is aresistive element and said reactive element is an inductive element.

'4. A waveform generator according to `claim 1, wherein said load isconnected in series `relationship with said collector.

5. A waveform generator according to claim 1, wherein said switchcomprises semiconductor switching means.

6. A waveform'generator comprising: `a first transistor having anemitter, a collector Iand a base, the base-emitter path of saidtransistor having a resistance which changes -inresponse to'a changingforward-biasing voltage applied between said emitter and said base and acurrent gain which changes in response to a change `in the collectorcurrent ot said transistor; a direct-currentaconductive impedanceelement; means connecting said impedance element between said emitterand said base; a reactive element; a switchY comprising a secondtransistor having an emitter, a collector anda base; means lforsupplying a unidirectionalvoltage; means connecting said impedanceelement, the.ernittercollector path of said second transistor, saidreactive element `and said voltage-supplying means in seriesrelationship; and a load coupled to the emitter-collector path of saidfirst transistor; said unidirec- .tional voltage having such polarity-as to supply to said impedance element, upon closure of said switch, arst current portion sensed to develop across said impedance element avoltage which forward-biases said base-emitter pathof said `firsttransistor, thereby enablinga second current portion to flow from saidvoltage-supplying means through said base-emitter path ofsaid firsttransistor, said unidirectional voltage having a value sufficiently highthat said forward-biasing voltage can change by an amount producingsubstantial changes respectively in said resistance of said base-emitterpath, said collector current and said lcurrent gain of said firsttransistor, Ysaid impedance element having an impedance value such thata substantial chang-ein said resistance of said base-emitter path ofsaid iirst transistor causes a substantial change in the ratio of therespective intensities of said iirst and said second current portions,.and means for closing said switch for a time sufficiently long for saidforward-biasing voltage to change in value by `an amount producing saidrespective substantial changes in said resistance of said base-'emitterpath, said collector current and said current gain of said firsttransistor.

7. A waveform generator according to claim 6, wherein said base-emitterpath Vof saidirst transistor has 1a resistance which decreases inresponse to an increasing forward-biasing voltage appliedbetween saidemitter and said base of said first transistor and a current gain whichdecreases in response to anV increase `in said collector current of saidfirst transistors, said direct-current-conductive impedance element isaresistive element,` and said reactiveelement is an inductive element.

8. A waveform generator according to claim?, wherein said means forclosing said switch comprise means for supplying to said baseof saidsecond transistor a v switching potential which forward-biases Vthebase-emitter path of said second transistor during periodicallyrecurring times of `at least said suiiicient length and reversebiasessaid base-emitter; path of `said second transistor during the` timesbetween successive ones of said periodically recurring times.

9 10 9. A waveform generator according to claim 7, Where- ReferencesCited by the Examiner in said inductive element has an induotancesuciently UNITED STATES PATENTS small compared to the resistance of thepanallel combination of said resistive element and said base-emitter2934657 4/60 Rack 307- 88'5 2,939,040 5/60 Isabeau 307-885 X path ofsald first transistor that the Waveform of the cur- 5 3 089 964 5/63Bruce et al 30,7 88 5 rent passing through said inductive element whensaid switch is closed is substantially curved. JOHN W. HUCKERT, PrimaryExaminer.

1. A WAVEFORM GENERATOR COMPRISING: A TRANSISTOR HAVING AN EMITTER, ACOLLECTOR AND A BASE, THE BASE-EMITTER PATH OF SAID TRANSISTOR HAVING ARESISTANCE WHICH CHANGES IN RESPONSE TO A CHANGING FORWARD-BIASINGVOLTAGE APPLIED BETWEEN SAID EMITTER AND SAID BASE AND A CURRENT GAINWHICH CHANGES IN RESPONSE TO A CHANGE IN THE COLLECTOR CURRENT OF SAIDTRANSISTOR, A DIRECT-CURRENT-CONDUCTIVE IMPEDANCE ELEMENT, MEANSCONNECTING SAID IMPEDANCE ELEMENT BETWEEN SAID EMITTER AND SAID BASE, AREACTIVE ELEMENT, A SWITCH AND MEANS FOR SUPPLYING A UNIDIRECTIONALVOLTAGE, MEANS CONNECTING SAID IMPEDANCE ELEMENT, SAID SWITCH, SAIDREACTIVE ELEMENT, AND SAID VOLTAGE-SUPPLYING MEANS IN SERIESRELATIONSHIP, AND A LOAD COUPLED TO THE EMITTER-COLLECTOR PATH OF SAIDTRANSISTOR, SAID UNDIRECTIONAL VOLTAGE HAVING SUCH POLARITY AS OF TOSUPPLY TO SAID IMPEDANCE ELEMENT, UPON CLOSURE OF SAID SWITCH, A FIRSTCURRENT PORTION SENSED TO DEVELOP ACROSS SAID IMPEDANCE ELEMENT AVOLTAGE WHICH FORWARD-BIASES SAID BASE-EMITTER PATH OF SAID TRANSISTOR,THEREBY ENABLING A SECOND CUR-